In the last few hundred years, the most commonly used mechanical lock systems were developed in several paths, e.g., warded lock, lever lock, and cylinder lock. Among them, the paths of warded lock and lever lock systems have the advantage of protecting the locking mechanism with a strong outer cover against destructive entry. However, they are easier to be bypassed in comparison with a cylinder lock system, which has double detainer pins or wafers. With the double-acting detainer locking principle, the cylinder lock system has been developed and used most extensively because of its high security against bypass. It has been used in wide variety of types of mechanical locks, and dominates the current market.
The basic design of a cylinder lock system has a key cylinder which is mounted rotatably within a cylindrical bore in a housing. A set of detainers (mostly following the double-acting detainer locking principle), e.g., tumbler pins, wafers, et cetera are inserted into the bores of both the key cylinder and its surrounding housing, straddling the shear line (the cylindrical surface or end surface of the cylinder). These detainers prevent the cylinder from rotating about its longitudinal axis, if a correct key is not inserted into the keyway of the cylinder. The insertion of a correct key will move all the detainers to appropriate locations clear of the shear line, freeing the cylinder to rotate through application of a rotational moment to the longitudinal axis of the key, and that of the cylinder. In turn, with or without the help of other connecting mechanical components, the rotating cylinder transmits the action to the bolt or shackle to open the lock. In a sense, the housing is considered a non-mover; and, the key is used to turn the cylinder which is the mover, and to move the bolt or shackle of the lock.
Many methods and tools for lock bypass have been developed. Most commonly used cylinder locks can be bypassed by picking, bumping, impressioning, or decoding. An attacker has at his disposal various tools: pick, pick gun, wire snap pick, “999 rapping key” or bump key, decoder with fine shim wire (such as John Falle's Pin Lock Decoder, globally accepted by law enforcement and intelligence agencies, shim to be inserted between the padlock shackle and the lock body), or other specially designed tools to manipulate and decode locks. The “999 rapping key” is one of the favorite bypass tool because, a single key can be used to open many locks which have the same keyway and pin spacing. Since the “999 rapping keys” can be made inexpensively with recycled keys, criminals can invest very little money to buy just a few of them from many brands of lock to bypass numerous locks. Manufacturers of high security locks counter these bypass attacks with improvements to all components in the cylinder lock system, e.g., mushroom pin, spool pin, serrated pin, long tumbler pin occupying the upright channel, sidebar, rotating pin, telescoping pin, angularly bitted key, laser track on key blade, et cetera. Most improvements have complex design, requiring extremely precise machining, some on tiny parts, and very expensive production.
Furthermore, most cylinder locks, including some high security locks, can be compromised by destructive entry methods, some rather easily. For instance, since the cylinder is the mover, usually it can be shielded only partially from attack by outside force. Using a drill or mill, an attacker can easily destroy the cylinder, pins, wafers, et cetera of many locks by drilling through the keyway, the exposed cylinder, or the shear line. Some cylinders are protected with a small hardened steel pin near the keyway entrance to counter this kind of attack. However, this type of protection is weak in comparison with a strong outer facing. In addition, the commonly used “screw driver and wrench” attack method, described in Marc Weber Tobias, J. D., “Locks, Safes, and Security,” Vol. 1 and 2, 2nd Edition, 2000, Charles C. Thomas Publisher, LTD., and Marc Weber Tobias, J. D., “High Security Lock Standards and Force Entry: a Primer,” http://download.security.org/forced_entry—2007.pdf, can destroy and open easily most locks with cylinder and shell housing design because the attacker has leverage advantage to overcome the resistance. One of the main functions of the cylinder is to be turned by the key and transmit the operational torque to other components of the lock. The attack force to destroy the restrainers (cylinder, pin, wafer, et cetera) enters the lock through the same path used by the operational torque. Therefore, there is no way to avoid or protect the restrainers from an overwhelming attack force. In addition, the complex design of the cylinder to guard against bypass can introduce delicate components and will fail defend against destructive entry attack. In some cases, intricate design requires machining off more material from the cylinder and weakening it as a result. Most high security cylinder locks contain parts which are complicated and machined precisely, some very tiny—for example, the machining of the pin bores, slot in the cylinder for side bar, keyway, housing of the cylinder, et cetera. These requirements steer the production to the use softer metals to make the cylinder and its housing. Unfortunately, the small cylinder, full of bores and with the keyway opened to outside, is the main target of destructive entry such as “screw driver and wrench” attack, drilling, thermal attack, chemical attack, et cetera.